The invention relates to stress reduction in combustion cases in gas turbine engines.
FIG. 1 illustrates the outer surface of a segment 3 of a combustor case used in a gas turbine engine. The overall case is generally cylindrical, or conic, and the conic/cylinder is formed by extending segment 3 around axis 6, as indicated by arrows 9. FIG. 2 illustrates the inner surface 12 of the segment 3 of FIG. 1.
Apertures or holes 15 are formed within the case, for various purposes, such as delivery of fuel to combustors (not shown) within the case. The apertures penetrate the case in regions where the material of which the case is constructed is dimensionally thin. The thin material provides a less-than-optimal attachment point for external structures, such as a fuel-delivery tube. Further, the apertures themselves act as stress-risers, and increase stress concentrations in the already thin material surrounding them.
In order to dissipate the stress concentrations, strengthen the region surrounding the apertures 15, and to provide a convenient flange for attachment of tubing or sensors, bosses 18 are provided. FIG. 3 illustrates a boss 18 in schematic, cross-sectional view.
Traditionally, as indicated in FIGS. 1 and 2, a separate boss 18 is provided for each individual aperture 15. Further, for each aperture, two bosses are provided: a boss 18 on the outer surface, as in FIG. 1, and a boss 18 on the inner surface, as in FIG. 2.
The individual bosses on the inner surface increase manufacturing costs. In one manufacturing approach, a complex milling set-up must be used, partly because the diameter of the case is small compared with the size of an ordinary vertical mill. In another approach, Electro Chemical Machining, ECM, is used.
It is desired to eliminate, or reduce, the complexity and expense of the traditional approach to manufacturing the case of FIGS. 1 and 2.
In one form of the invention, individual bosses for individual apertures on the inner surface of a combustion case are eliminated, and replaced by a continuous circumferential band having a thickness similar to that of the eliminated bosses. A circumferential array of T-shaped slots is generated within the band, on the inner surface of the case. These T-shaped slots separate the continuous band into individual areas of reinforcement bosses, each of which surrounds multiple apertures.